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fix(devices): USB printing dropped the job tail — chunked write loop

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Field bug (ICS XP-K200L over USB): text printed, barcode + cut missing;
same bytes over TCP fine. sendRawUsb did ONE write() on an O_NONBLOCK
usblp fd and never checked bytesWritten — the kernel accepts only what
fits the printer's ~8 KB USB buffer and returns a short write, so the
tail of any job bigger than one buffer (the barcode mid-payload, the cut
at the end) was silently discarded. The regular-file test stand-in can't
short-write, which is why tests never caught it.

writeAllUsb now pushes 4 KB chunks until every byte is accepted,
continues after partial writes, retries EAGAIN/zero-byte with a short
pause, and fails at the deadline with an (N/M bytes) diagnostic. Driven
by fake-handle tests (short writes, EAGAIN interleave, wedged-printer
timeout, non-EAGAIN passthrough).

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V
This commit is contained in:
Julian Çuni
2026-07-06 12:36:14 +02:00
parent 7ef332999e
commit 81bc2e357c
3 changed files with 140 additions and 6 deletions
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wiki/concepts/printer-usb-transport.md
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@@ -2,7 +2,7 @@
type: concept
tags: [parking, device, printer, transport, usb, escpos, provisioning]
sources: []
updated: 2026-06-24
updated: 2026-07-06
status: settled
---
@@ -87,5 +87,28 @@ The setup UI offers a **Connection** select (Network / USB) + a **USB device** p
to the node and reports ready/offline). The on-hardware confirmation + the udev/usblp provisioning
are pending (open-questions #14).
## Field bug — the NONBLOCK partial-write truncation (found + fixed 2026-07-06)
First on-hardware USB test (ICS XP-K200L, an ESC/POS clone): over TCP it printed + cut fine; over
USB it printed the ticket's TEXT but **no barcode and no cut**. Root cause was in OUR transport,
not the printer: `sendRawUsb` opened the node with `O_NONBLOCK` and issued ONE `write()` for the
whole job. On a non-blocking usblp fd the kernel accepts only what fits the printer's USB buffer
(~8 KB) and returns a **short write**; the old code never checked `bytesWritten`, closed the
handle, and silently dropped the tail — which is exactly where the barcode (mid-payload) and the
CUT (last bytes) live. Small jobs fit one buffer, hence "text prints fine". The regular-file test
stand-in can't short-write, so tests never caught it.
Fix: `writeAllUsb` — chunked loop (4 KB, safely under the usblp buffer) that continues after
partial writes, retries `EAGAIN`/zero-byte writes with a short pause, and fails at the caller's
deadline with a `(N/M bytes accepted)` diagnostic. Driven by fake-handle tests (short writes,
EAGAIN interleave, wedged-printer timeout, non-EAGAIN passthrough) since a real file can't
reproduce the char device's behaviour.
> Driver-choice note for this clone: the ICS XP-K200L does NOT serve the Rongta `/prn_stat.htm`
> status page (checked on hardware at 10.0.10.11 — print socket 9100 open, status page absent),
> so on NETWORK the honest driver is **cashino** (reachability-only monitoring); under `rongta`
> the monitor would mark a perfectly working printer offline/degraded. Over USB the two drivers
> behave identically (reachability floor), so either works post-fix. See [[rongta-printer]].
Related: [[rongta-printer]], [[printer-status-monitoring]], [[printer-roles-failover]],
[[appliance-provisioning]], [[network-isolation]], [[technology-stack]].